The contamination of the environment with aqueous solutions of heavy metals from industrial waste stream remains a serious problem. In spite of significant advances in the treatment of such effluent in recent years, no completely satisfactory method for the removal of such toxins from industrial effluent yet exists. One area of particular concern is the aqueous run-off from mining operations, including so called "acid mine drainage." The effluent from mining operations frequently contains a complex mixture of heavy metals such as copper, nickel, lead, zinc, etc. at concentrations well above the acceptable regulatory limits, but too low to be economically recovered by conventional processes. These streams which are usually at a low pH, also contain large quantities of other, less toxic metals such as iron and calcium.
Currently, the method most widely used to remove toxic metals from effluent streams involves raising the pH of the solutions to the level at which the metal hydroxides are least soluble, usually between 9 and 11, so that they can be removed by precipitation. The best technology hitherto in use employs hydrated lime as an alkali source, with precipitation of the metal hydroxides in large clarifiers facilitated by additional sedimentation aids such as ferric sulfate and organic polymers. This treatment results in the recovery of large quantities of a voluminous sludge consisting of a mixture of the metal hydroxides admixed with calcium sulfate (gypsum). Since the metals are unrecoverable from this gypsum matrix, the sludge is then transferred to a landfill site, or returned to the tailings pile from which the metals had originally emanated. This procedure is undesirable for a number of reasons. For one, the steadily rising costs of landfill make the disposal of toxic sludge ever more expensive. The second problem is that the metals are not permanently removed from the environment. This is because the chemical environment within the landfill itself is usually unstable, and subject to a steady decline in pH. As this occurs the metal hydroxides re-dissolve and re-enter the environment, requiring yet another treatment. Since mine tailings and landfill sites are likely to remain in place for many centuries, such an ongoing cycle is clearly unacceptable.
Another factor contributing to the desirability of extracting the heavy metals from the effluent is the fact that in their pure form the metals are of considerable economic value. Not unexpectedly, much attention has been given in recent years to methods which might allow the recovery of the dissolved metals from solution. Some techniques which have been taught to achieve this end include the use of membrane filtration, or electrochemical methods [L. L. Tavlarides et al. Separation Sci & Technology 22: 2-3 (1987)] These methods, however, suffer the disadvantage of high operational cost, and are inadequate for the large volumes of liquid commonly encountered in mine effluent streams. Another method which has enjoyed some success in the removal (but not recovery) of the toxic metals involves insolubilization of the gypsum sludge by the formation of a cementitious matrix, the so-called "Chemfix" process. (R. B. Pojesek Chem Eng. 86, Aug. 13, 1979; P. G. Lawrence Chem-fix Inc. Report 1980, Pittsburgh, Pa.). This method has not however received widespread application since the treatment is not only expensive, but the long term validity remains unproven. The use of soluble alkali silicates for the removal of heavy metals from solution is described by J. S Falcone (ACS Symposium Ser. 194 Am. Chem Soc. New York, 1982), but this too results in the formation of a complex precipitate from which the metals cannot be economically extracted.
Various chemical methods to recover metals from waste streams by ion exchange have been described. Thus zeolite (M. J. Zamzow et al. Sep. Sci. Technology 25: 13-15 (1990) 1555-69), quartz (T. W. Healy et al. Adv. Chem. Ser 79 (1968) 62) and alumina (M. Uberoi and F. Shadman Prep. Pap. Am. Chem Soc. Div Fuel Chem 4 (1991) 36) have all been recommended for this purpose. Although each of these methods offers the promise of recovering the metals from solution, they all require relatively high concentrations of metals in solution to be effective. The German Patent disclosure DE 42 44 258 A1 (Jun. 23, 1994; Grace GmbH) on the other hand teaches that silica gel can be used to concentrate cadmium in solution from quite dilute solutions, but this method is relatively slow, and suffers from a number of other disadvantages for which silica gel is well known.
The ability of silica gel to remove metals from solution by selective adsorption has been extensively described, and the adsorption and desorption of a wide range of metals under different pH conditions has been studied by D. L. Dugger et al. (J. Phys. Chem. 68 (1964) 757060), R. O. James and T. W. Healy (J. Coll. Interface Sci. 40 (1972) 65-81), V. F. and J. Galba Coll (Czech Commun. 32 (1967) 3, 530-6), and P. W. Schindler et al. (J. Coll Interface Sci. 55 (1976) 469-75). A theoretical discussion of the adsorption of dissolved metals by silica is also to be found in R. K. Iler ("The Chemistry of Silica," New York: John Wiley & Sons, 1979). One of the problems of using silica gel as an ion exchange medium, however, is that being a weak acid, the pH of the solution from which the metals are removed declines as the metals are adsorbed onto the silica gel. This has the consequence that as the process proceeds desorption begins to occur. Even if the pH were to be artificially controlled in order to effect selective adsorption of metals from solution (as would be obvious to those skilled in the art), another problem arises due to the fact that silica gel is physically quite fragile and expensive. The handling of the product which is required to facilitate the process can frequently lead to destruction of the gel, at considerable financial cost.
Various recent disclosures have sought to avoid some of these drawbacks by adsorbing various organic and inorganic ion exchange materials onto either alumina or silica gel, which such cases are treated as an inert substrate. Thus the German patent disclosure DE 3823957 A1 (Jan. 18, 1990, R. Ballhorn) teaches that heavy metals can be removed from solution by means of calcium phosphate adsorbed onto silica gel. Similarly Schlapfer (C. W. Schlapfer, U.S. Pat. No. 5,102,640 Apr. 7, 1992) describes a process for the removal of metal ions from solution by means of dipicolylamine bound to a silica gel surface while G. Giraudi et al. (Annali di Chemica 74 (1984) 307-13) present a method of concentrating metal ions using pyridylazo naphthol adsorbed on silica gel. A summary of some other complex forming reagents supported on silica gel which have been tried for this purpose was published K. Terada (analytical Sciences 7 (1991) 187-98). Such methods, although valuable for analytical purposes are, however, inadequate for use with industrial waste and process streams. The coated silica gel materials described in these articles are both too expensive and fragile for the numerous cycles required to allow this method to compete with alternate treatments such as lime.